Antiarrhythmic Drugs

Fast-response cells have a strongly negative resting membrane potential of approximately negative 90 millivolts, maintained by outward potassium current. Their action potential is divided into five phases, each governed by specific ion movements.

Phase 0, rapid depolarization, occurs when the membrane reaches threshold and voltage-gated sodium channels open rapidly, driving a large inward sodium current. This fast upstroke produces the rapid conduction velocity characteristic of working ventricular muscle and the His-Purkinje system. Class I antiarrhythmic drugs act here by blocking sodium channels and slowing conduction.

Phase 1, early repolarization, begins as sodium channels inactivate within milliseconds. A brief outward potassium current drives the membrane toward the plateau. Phase 2, the plateau, is sustained by a balance between inward calcium current through L-type calcium channels and outward potassium currents. This calcium entry triggers contraction. Class IV agents — the non-dihydropyridine calcium channel blockers — and amiodarone act at the calcium channel during this phase.

Phase 3, rapid repolarization, occurs as calcium channel current declines and outward potassium currents dominate, restoring the resting potential. Class III agents prolong this phase by blocking the repolarizing potassium channels, which is the basis for their action potential duration prolongation and the mechanism of drug-induced QT interval prolongation. Phase 4 is electrically stable in non-pacemaker cells; the sodium-potassium pump restores ionic gradients, and an inward-rectifying potassium current holds the membrane at rest.

Nodal cells operate on fundamentally different ionic machinery. Their resting membrane potential is less negative — approximately negative 60 to negative 70 millivolts — which keeps fast sodium channels largely inactivated. As a result, nodal cell depolarization depends on calcium current through L-type calcium channels rather than on sodium channels. This produces a slow upstroke and slow conduction velocity through the nodes.

The clinically defining feature of nodal cells is spontaneous phase 4 depolarization, the pacemaker potential. This gradual drift toward threshold is driven by inward current through channels that open on hyperpolarization (the funny current, or If), combined with declining outward potassium current and late-activating calcium current. The sinoatrial node fires fastest and drives the heart rate under normal conditions; the atrioventricular node follows as a subsidiary pacemaker.

Because nodal automaticity depends on calcium current and adrenergic input to the If channel, these cells are the primary targets for rate control: beta-blockers reduce the slope of phase 4 by decreasing adrenergic stimulation of If, while non-dihydropyridine calcium channel blockers (verapamil, diltiazem) reduce calcium-dependent upstroke and atrioventricular nodal conduction. Class II (beta-blockers) and Class IV agents are the nodal-targeting drug classes.

Class I agents block voltage-gated fast sodium channels in fast-response tissue, slowing the rate of phase 0 depolarization and reducing conduction velocity. The three subclasses differ in their speed of channel binding and unbinding, which produces distinct clinical profiles.

Class Ia agents — quinidine, procainamide, and disopyramide — have intermediate binding kinetics. They slow conduction, prolong action potential duration, and produce QT interval prolongation on the electrocardiogram. They carry risk of torsades de pointes because of this action potential duration prolongation.

Class Ib agents — lidocaine and mexiletine — bind and dissociate rapidly. They shorten action potential duration and are selectively active in diseased or ischemic tissue that fires rapidly. Because they dissociate so quickly, they have minimal effect on normal tissue at resting heart rates, making them relatively safe for acute ventricular arrhythmia management in ischemic settings.

Class Ic agents — flecainide and propafenone — bind slowly and dissociate very slowly, producing marked slowing of conduction with QRS widening on the electrocardiogram but minimal change in action potential duration. This profound conduction slowing is effective in atrial arrhythmias in structurally normal hearts but carries serious risk in the presence of structural heart disease.

Class II agents competitively block beta-adrenergic receptors, reducing sympathetic drive to the heart. Their antiarrhythmic effects are predominantly on nodal tissue and on arrhythmias driven by catecholamine excess. They decrease the slope of spontaneous phase 4 depolarization in pacemaker cells, prolong atrioventricular nodal conduction and refractoriness (increasing the PR interval), and suppress delayed afterdepolarization-mediated triggered activity in catecholamine-sensitive arrhythmias. Beta-blockers are also anti-ischemic, reducing arrhythmia substrate by decreasing myocardial oxygen demand.

Class III agents block the repolarizing potassium channels responsible for phase 3, prolonging action potential duration and the effective refractory period. The net electrocardiographic effect is QT interval prolongation. By prolonging refractoriness, these agents eliminate the excitable gap in re-entrant circuits — the pharmacologic rationale for rhythm control in both atrial and ventricular arrhythmias.

Amiodarone is the most widely used Class III agent but is mechanistically unusual: it blocks sodium channels, calcium channels, beta-adrenergic receptors, and potassium channels — effectively spanning all four Vaughan Williams classes. This multi-channel activity makes it the broadest-spectrum antiarrhythmic drug available. Its extraordinary volume of distribution and elimination half-life of 40 to 55 days necessitate loading protocols and produce toxicities that develop over months to years.

Sotalol has both Class III potassium channel blocking activity and significant beta-adrenergic receptor antagonism. Dofetilide and ibutilide are more selective potassium channel blockers used for rhythm control in atrial fibrillation. A critical property of Class III agents is reverse use-dependence: their action potential duration prolongation is paradoxically greatest at slow heart rates, which is also the condition most favorable for early afterdepolarization formation — meaning their proarrhythmic risk is highest precisely when they provide the least protection at rapid rates.

Class IV agents block L-type calcium channels. Their primary antiarrhythmic targets are the sinoatrial and atrioventricular nodes, where calcium current drives the upstroke and contributes to the pacemaker potential. Verapamil and diltiazem — the two antiarrhythmic agents in this class — reduce atrioventricular nodal conduction velocity and prolong atrioventricular nodal refractoriness, making them effective for ventricular rate control in atrial fibrillation and for terminating atrioventricular nodal-dependent re-entrant tachycardias.

Class IV agents have minimal effect on fast-response ventricular tissue at therapeutic concentrations. This explains both their utility for supraventricular arrhythmias and a critical contraindication: verapamil and diltiazem must not be used in wide-complex tachycardias presumed to be ventricular tachycardia, where their negative inotropic and vasodilating effects can cause hemodynamic collapse without providing antiarrhythmic benefit.

Several agents do not fit neatly into a single class. Amiodarone has Class I, II, III, and IV properties simultaneously. Sotalol has Class II and Class III activity. Assigning a single class label to these agents misrepresents their pharmacology and can create a false sense of predictability.

Reverse use-dependence is a particularly important limitation for Class III agents. Their action potential prolongation is paradoxically greatest at slow heart rates — the condition most conducive to early afterdepolarization formation and torsades de pointes. This means the proarrhythmic risk of Class III drugs is highest at the heart rates where they provide the least anti-tachycardia protection.

The classification is based on drug mechanism, not arrhythmia mechanism. Knowing that a drug is Class Ic does not indicate which arrhythmias it should or should not treat. Clinical decision-making requires knowledge of both the drug class and the arrhythmia substrate.

Structural heart disease constrains drug selection more than any other single factor. Class I agents that slow conduction can convert a non-sustained arrhythmia into a sustained or fatal one in scarred myocardium by facilitating stable re-entrant circuits. This is why Class Ic agents are contraindicated in structural heart disease, a restriction established by the Cardiac Arrhythmia Suppression Trial and still fundamental to antiarrhythmic prescribing today.

Proarrhythmia is class-specific and predictable from mechanism. QT interval prolongation and torsades de pointes risk arise from Class Ia and Class III agents. Negative inotropic effects are most pronounced with Class Ic agents and verapamil. Atrioventricular block is a risk with Class II, Class IV, adenosine, and digoxin. Anticipating these effects based on mechanism — rather than encountering them as surprises — is the goal of mechanistic pharmacology training.

Combination antiarrhythmic therapy does not reliably produce additive benefit but does produce additive toxicity. Combining a Class Ia agent with a Class III agent, for example, compounds QT prolongation risk substantially. Polypharmacy in this drug class requires explicit attention to the cumulative proarrhythmic burden.